Frailing is the
strumming technique (as opposed to
fretting) of playing a stringed instrument (usually a 5-string
banjo), whereby the nail of the index or middle finger is used to strike downward on a string, sounding the note. It is used in conjunction with a very specific rhythm (called "bum-ditty") and thumb technique of playing the shorter 5th string of the banjo.
The "bum-ditty" rhythm is a quarter note (bum) and 2 eighth notes (dit & ty). The bum is a single string struck with the index finger, the dit is a brushing of multiple strings with the index finger, and the ty is the sounding of the 5th string with the thumb.
Bum is the melody note (of which the left hand can hammer-on or pull-off the string to create more intricate runs), the dit and ty are rhythm accompaniment. The effect is akin to a player accompanying herself.
Using this technique, a banjo player can suitably accompany another instrument AND perform solos without changing the mode of playing.
Note - The term 'frailing' is related to/confused with/used interchangeably with the term clawhammer, which is an extension of the frailing technique where the thumb is not limited to striking the shorter 5th string of the banjo.